The usual advantage of microwave (MW) assistance is making organic reactions faster and more efficient. In this article we present reaction types from organophosphorus chemistry, when MW-assisted transformations (e.g. the direct esterification and alkylating esterification of phosphinic acids) may be promoted by suitable catalysts, or vice versa, when a catalytic reaction is enhanced by MW irradiation (e.g. the Arbuzov reaction of aryl halides), and when catalysts may be omitted or simplified under MW irradiation as shown by the alkylation of active methylene containing P=O substrates/the Kabachnik–Fields reaction/deoxygenation of phosphine oxides, and the Hirao reaction, respectively.
The spread of the microwave (MW) technique has had a great impact also on organophosphorus (OP) chemistry. Guenin was who summarized the initial results . The senior author of this paper and his co-workers made efforts to make use of the MW tool in the synthesis of P-heterocycles and other phosphinic-, phosphonic- and phosphine oxide derivatives , , , , , , , . The stress was placed on green chemical aspects , . Beside elaborating efficient syntheses characterized by short reaction times and good yields, it was also possible to promote otherwise reluctant transformations, such as the inverse-Wittig type reaction of 2,4,6-trialkylphenylphosphine oxides with dialkyl acetylenedicarboxylates to afford β-oxophosphoranes , , . Another challenge was for us to evaluate the scope and limitations of the use of the MW technique in organic syntheses, and to model the effect of MWs , , .
In this article, we wish to summarize our results on the advantageous use of catalysts in MW-assisted reactions, and on the beneficial effect of MWs in a catalytic transformation. Furthermore, we show cases, when catalysts may be simplified, or even omitted under MW irradiation.
MW-assisted direct esterification of cyclic phosphinic acids – the beneficial effect of an ionic liquid additive
We found that cyclic phosphinic acids, such as 1-hydroxy-3-phospholene oxides (1), 1-hydroxyphospholane oxides (2) and a 1-hydroxy-1,2,3,4,5,6-hexahydrophosphinine oxide (3) underwent direct esterification with simple alcohols used in a 15-fold excess at 220–235°C under MW irradiation to give the corresponding phosphinates (4–6) (Scheme 1) , , .
It was experienced that the esterifications with longer carbon atom chain (and hence less volatile) alcohols were more efficient giving the corresponding phosphinates (4–6) in yields of 60–95%. The reaction times were mostly around 1–5 h. We also observed that the MW-promoted thioesterifications and amidations led only to incomplete conversions , .
These days, ionic liquids are found applications as “green” solvents , but they might be proven even more useful as catalysts or additives . We experienced that in the presence of 10% of [bmim][PF6], the MW-assisted esterification of 1-hydroxy-3-methyl-3-phospholene oxide (1A) could be performed at a lower temperature (180°C), in a shorter reaction time (0.5 h), and in a high conversion (90%) allowing a good yield (83%), as compared to the MW-assisted variation lacking an IL additive, in which case a conversion of 60% could be attained after an irradiation at 200°C for 2 h (Scheme 2) , . For comparison purposes, results of the inefficient thermal experiments are also shown in Scheme 2.
The ionic liquid catalysis was then utilized in the esterification of phosphinic acid 1A with other alcohols like ethanol, propanol, pentanol and isooctanol (Table 1). The effect of [bmim][PF6] was indeed significant in promoting higher conversions and yields at lower temperatures and after shorter reaction times. It is an especially valuable result that the esterifications with volatile alcohols like EtOH and PrOH became much more efficient , .
|R||[bmim][PF6] (%)||T (°C)||p (bar)||t (h)||Conversion (%)||Yield (%)|
|i Oct (e)||–||220||3||1||100||76|
|i Oct (e)||10||180||2||0.33||100||84|
And what is more, the [bmim][[PF6]-promoted protocol could be extended to the esterification of dimethyl-hydroxyphospholene oxide 1B, 1-hydroxyphospholane oxides 2A and 2B, and a hydroxy-1,2,3,4,5,6-hexahydrophosphinine oxide (3) using pentanol as the alcohol (Table 2). The significant effect of the IL additive could again be observed , .
|Model reaction||[bmim][PF6] (%)||T (°C)||p (bar)||t (h)||Yield (%)|
MW-assisted alkylating esterification of cyclic phosphinic acids – a phase transfer catalytic support
The synergism of MW irradiation and phase transfer catalysis was observed in the alkylating esterification of cyclic phosphinic acids, such as 1-hydroxy-3-phospholene oxides (1), 1-hydroxyphospholane oxides (2) and 1-hydroxy-1,2,3,4,5,6-hexahydrophosphinine oxide (3) in the presence of K2CO3 under solvent-free conditions using n-alkyl halides (Scheme 3). The phosphinates (4–6) were obtained in yields of 73–95% , , . In the case when an alkyl halide with increased reactivity, such as benzyl bromide was used, MW was beneficial alone, and there was no need for a catalyst.
Catalytic Michaelis–Arbuzov reactions – MW assistance
The Arbuzov reaction is a suitable approach for the preparation of arylphosphonates . Special catalytic protocols are necessary to overcome the low reactivity of the aryl halides. The Ni-catalyzed reaction of aryl bromides with triethyl phosphite was further developed, and performed under MW irradiation. The phosphonates (7) could be prepared in yields of 67–90% (Table 3) .
Alkylation of active methylene-containing compounds – replacement of the phase transfer catalyst by MW irradiation
A typical case, where the catalyst may be substituted by MW irradiation is the alkylation of CH acidic compounds, such as tetraethyl methylenebisphosphonate (8), diethyl cyanomethylphosphonate (9), and diethyl ethoxycarbonylmethylphosphonate (10) (Table 4). In a solid–liquid phase accomplishment, potassium- or cesium carbonate was used as the base, and under MW conditions, there was no need for phase transfer catalyst. The monoalkylated products (11–13) were obtained in yields of 64–80% , , . A dialkylating protocol was also elaborated by us .
|R1||Starting material||R||M||T (°C)||Product||Yield (%)|
The Kabachnik–Fields reaction – substitution of the catalyst by MW irradiation
α-Aminophosphonates (14, Y=RO), and α-aminophosphine oxides (14, Y=Ph) may be synthesized by the solvent- and catalyst-free MW-assisted Kabachnik–Fields (phospha-Mannich) condensation of primary amines, oxo compounds and >P(O)H reagents, such as dialkyl phosphites and diphenylphosphine oxide. Earlier preparations applied special catalysts , , , , , which mean cost and environmental burden. It was found that under MW conditions, there is no need for any catalyst (Scheme 4) .
The use of heterocyclic amines, such as pyrrolidine, piperidine derivatives, morpholine and piperazine derivatives, or heterocyclic >P(O)H species (e.g. 1,3,2-dioxaphosphorine oxide) led to N-heterocyclic  and P-heterocyclic  α-aminophosphonates. 3-Amino-6-methyl-2H-pyran-2-ones could also serve as amino components in Kabachnik–Fields reaction with formaldehyde and dialkyl phosphites or diphenylphosphine oxide .
Primary amines are able to participate in bis(Kabachnik–Fields) condensations . In such cases, alkyl or arylamines were reacted with two equivalents of the formaldehyde and the >P(O)H species to give the bis(Z1Z2P(O)CH2)amines (15) (Scheme 5) , , . Most of the reactions could be carried out without the use of solvent.
As further developments, the bisphosphinoyl derivatives (15, Z1=Z2=Ph) were deoxygenated to bis(phosphines) that were converted to cyclic platinum complexes , , . α-, β- And γ-amino acids (or their esters) were also utilized as starting materials in the double Kabachnik–Fields condensation to provide the corresponding bis(phosphono- or phosphinoyl) products , .
As special bis(Kabachnik–Fields) reactions, paraphenylene diamine was reacted with two equivalents of substituted benzaldehydes and diethyl phosphite, or terephthalaldehyde was reacted with two equivalents of an arylamine and a P-reagent .
Deoxygenation of phosphine oxides – MW as a substitute for catalyst
As was mentioned in the part discussing the phospha-Mannich reaction , , , the reduction of phosphine oxides to phosphines is an important transformation that is usually realized by the use of silanes . We found that under MW and solvent-free conditions, there is no need to use any catalyst. Moreover, user-friendly silanes, such as tetramethyl-disiloxane (TMDS) or polymethoxysiloxane (PMHS) could also be applied , , , . Selected results are shown on the reduction of 1-phenyl-3-methyl-3-phospholene oxide 16 (Table 5). Deoxygenation is a usual protocol to make available racemic or optically active P-ligands from the corresponding P-oxide precursors , .
|Silane||Equivalents||Mode of heating||T (°C)||t (h)||Conversion (%)||Yield (%)|
The Hirao reaction – A “P-ligand-free” accomplishment under MW irradiation
The Hirao reaction involving the P–C coupling of a vinyl halide/aryl halide/hetaryl halide and a >P(O)H reagent has become an important tool in the synthesis of phosphonates, phosphinates and phosphine oxides , . We found that in the coupling reaction of dialkyl phosphites with bromobenzene, there was no need for the expensive Pd(Ph3P)4 catalyst, or for the combined addition of Pd(OAc)2 and a P-ligand, as Pd(OAc)2 also catalyses the Hirao reaction in the in the presence of the >P(O)H reagent. A MW-assisted solvent-free accomplishment was the best choice , . Arylphosphonates 18 were obtained in 69–93% yields (Scheme 6).
The similar reaction of alkyl phenyl-H-phosphinates with bromobenzene led to alkyl diphenylphosphinates (19) (Scheme 7/(1)), while the reaction of secondary phosphine oxides with bromoarenes afforded phosphine oxides (20) (Scheme 7/(2)).
Furthermore, we found that the MW-assisted “P-ligand-free” protocol worked also with NiCl2 as the catalyst in the synthesis of arylphosphonates (18), alkyl diphenylphosphinates (19) and diaryl-phenylphosphine oxides (20, Z=Ar). In these variations, acetonitrile had to be used due to the heterogeneity of the reaction mixtures .
The P–C coupling of halobenzoic acids and diarylphosphine oxides could be performed in the absence of any catalyst, in the presence of K2CO3 in water under MW conditions (Scheme 8) .
As a matter of fact, in the “P-ligand-free” cases outlined above, the trivalent tautomer form of the >P(O)H reagent acts as the ligand .
The classical catalytic cycle for the Hirao reaction involves oxidative addition of the aryl halide to the Pd(0) complex (22) to give Pd(II) adduct 23, the change of ligands leading to key intermediate 24, and finally the reductive elimination affording the product (ArP(O)Y2) (Fig. 1).
The 23→24 transformation may be refined assuming the complexation of the trivalent form of the Y2P(O)H reagent, and that that this step is followed by deprotonation (Scheme 9).
As regards the interaction of Pd(OAc)2 and the >P(O)H species, Buono et al. claimed the formation of dimer type complexes (26) shown in Scheme 10 , .
It is noteworthy that the formation of an analogous dimer (27) was observed in the reaction of PdCl2(MeCN)2 with tert-butyl-phenylphosphine oxide (Scheme 11).
Theoretical calculations suggested that the reaction of Pd(OAc)2 with diphenylphosphine oxide or dimethyl phosphite cannot give the corresponding dimer type complexes 28, as their formation from the corresponding monomers is highly unfavorable marked by the enthalpy change of +82.3 kJ mol−1 and +20.7 kJ mol−1, respectively (Scheme 12) .
However, the analogous formation of the chloro-containing dimer type complexes 29 from the corresponding monomers was justified, as their formation is exothermic as characterized by enthalpy changes of –118.9 kJ mol−1 and –153.8 kJ mol−1, respectively (Scheme 13) .
Our idea was that the interaction of the Y2P(O)H species and Pd(OAc)2 results in eventually Pd(PY2OH)2 complex (31). In the first part of this series of reactions, the Pd2+ ion of Pd(OAc)2 is reduced to Pd0 by reaction with Y2P(O)H, then the metal is complexed by two Y2POH units (Scheme 14). As a newer development, intermediate 30 could be pointed out by theoretical calculations (Fig. 2).
The formation of complex 31 was in full agreement with our experimental data. The best experiment was, when 1.3 equivalents of the >P(O)H species were applied together with 10% of the Pd(OAc)2 catalyst. (0.1 Equivalents served as the reducing agent, 0.2 equivalents provided the P-ligand, and one equivalent was consumed as the reagent ).
High level quantum chemical calculations were performed on the “P-ligand-free” Hirao reaction of bromobenzene and diethyl phosphite or diphenylphosphine oxide (Scheme 15).
In the first stage of the transformation, catalyst complex 30 undergoes oxidative addition of PhBr in three elemental steps to afford species 32 (see top of Fig. 3). Then Pd-complex 32 loses the bromide anion to give species 33 that by reaction with Y2POH (1′) affords complex 34. Deprotonation and reductive rearrangement via intermediate 35 and the corresponding TS, respectively, led to complex 36, whose decomposition furnished the P–C coupled product 31, and regenerated catalyst 30 (Fig. 2) .
The “P-ligand-free” approaches are environmentally-friendly as save costs and environmental burdens. Till data, the P-ligand-free variation may be the most attractive protocol for the Hirao reaction. The “green” accomplishments have been reviewed by us , .
The Pudovik reaction – no need for MW assistance, as triethylamine-catalysis is enough
α-Aryl-α-hydroxyphosponates and α-aryl-α-hydroxyphosphine oxides (37) were synthesized in a catalytic and solvent-free MW-assisted reaction involving the addition of >P(O)H species to aryl aldehydes (Scheme 16) .
Dialkyl phosphites were also added to the carbonyl function of α-ketophosphonates in the presence of diethylamine and in the absence of any solvent under MW conditions to result in the formation of dronate analogs, α-hydroxybisphosphonates , .
However, MW assistance is not definitely necessary in the case of Pudovik reactions. We found that in the presence of trimethylamine as the catalyst dialkyl phosphites add on the carbonyl group of benzaldehyde derivatives already in acetone at the boiling point. Moreover, the α-aryl-α-hydroxyphosphonates precipitate on cooling from the mixture, hence this method represents the “greenest” protocol for the preparation of α-aryl-α-hydroxyphosphonates .
To our surprise, the α-hydroxyphosphonates prepared could be easily transformed to the corresponding α-aminophosphonates by reaction with primary amines . The substitution reaction was promoted by the neighboring group effect of the adjacent P=O function.
In summary, typical cases were overviewed for the MW versus catalytic accomplishments. There are instances (esterification of phosphinic acids), when MW assistance is further promoted by a suitable catalyst, or the opposite situation, when a catalytic transformation (e.g. a hindered Arbuzov reaction) is enhanced by MW irradiation. An additional possibility is, when a catalyst may be simply omitted under MW conditions as represented by the phospha-Mannich reaction, special C-alkylations, or the reduction of phosphine oxides. Last but not least, it was found that under MW irradiation, the excess of the >P(O)H reagent may serve as the P-ligand, hence there is no need to add the usual P(III) species.
A collection of invited papers based on presentations at the 22nd International Conference on Phosphorous Chemistry (ICPC-22) held in Budapest, Hungary, 8–13 July 2018.
Funding: This project was sponsored by the National Research Development and Innovation Fund (K119202). NZK was supported through the New National Excellence Program of the Ministry of Human Capacities (ÚNKP-17-4-I-BME-133).
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